970 LHe JOURNAL VOTNGE OLOGY. 
posa slates as well. ‘The porphyrites are largely altered forms of sur- 
face lavas and tuffs, resembling andesite and in part basalt, and these 
rocks have been folded and compressed along with the sediments of 
the Calaveras and Mariposa formations. ‘The areas called amphibolite- 
schist on the geological map are chiefly metamorphic forms of these 
porphyrite-tufts. 
The formation of the later series, resting on these older rocks, that 
deserves most attention is called Auriferous gravel formation. These 
gravels which are found chiefly on the ridge-tops, were deposited in 
Neocene time by rivers. These old streams, as may be seen by inspect- 
ing the map, united into one trunk a little to the north of the Bear 
Mountains, and there found an outlet into the gulf that then filled the 
San Joaquin Valley. At many localities these old-river gravels have 
been profitably mined for gold. Forming a capping to the gravels are 
usually beds of volcanic material, chiefly andesite and rhyolite. 
The Calaveras formation is of economic importance as containing 
frequent gold-quartz veins and lenses of limestone. Most of the latter 
are noted on the geological map. 
The Mariposa formation affords a good roofing slate, but is chiefly 
remarkable as containing, in Amador county and in the north portion 
of Calaveras county, the quartz veins of the Mother Lode. 
The amphibolite-schist belts contain copper deposits and gold- 
quartz veins. In the southern part of Calaveras county, at Angel’s 
Camp, the Mother Lode lies to the east of the Mariposa slates and 
intersects a belt of amphibolite-schist. 
In the granite of the West Point area are numerous gold-quartz 
veins, the ores of which contain a larger per cent. of sulphurets than 
the ores of the Mother Lode mines, and such ores are called base. 
The serpentine areas contain chrome-iron deposits at numerous 
points. 
The tuffs overlying the gravels at Mokelumne Hill, Valley Springs, 
and other points, have been found to make good building stone. 
Sandstone quarries are worked in the foothills in beds of Tertiary age, 
and the deposits of the same age near [one afford large quantities of 
clay for pottery, and of coal. 
U.S. Geologic Atlas, Folio 12, Estillville, Kentucky; Virginia; Tennes- 
968, REO 
This folio consists of five pages of text by M. R. Campbell, 
geologist, a topographic map of the district (scale 1: 125,000), a sheet 
